linux-kernel - Re: Xen is a feature

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Message-ID: <20090607103547.GR31286@elte.hu>
Date:	Sun, 7 Jun 2009 12:35:47 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	George Dunlap <george.dunlap@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>,
	"jeremy@...p.org" <jeremy@...p.org>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Keir Fraser <Keir.Fraser@...citrix.com>,
	"gregkh@...e.de" <gregkh@...e.de>,
	"kurt.hackel@...cle.com" <kurt.hackel@...cle.com>,
	Ian Pratt <Ian.Pratt@...citrix.com>,
	"xen-users@...ts.xensource.com" <xen-users@...ts.xensource.com>,
	ksrinivasan <ksrinivasan@...ell.com>,
	"EAnderson@...ell.com" <EAnderson@...ell.com>,
	"wimcoekaerts@...mekes.net" <wimcoekaerts@...mekes.net>,
	Stephen Spector <stephen.spector@...rix.com>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"npiggin@...e.de" <npiggin@...e.de>
Subject: Re: Xen is a feature


* Avi Kivity <avi@...hat.com> wrote:

> Ingo Molnar wrote:
>>> There is in fact a way to get dom0 support with nearly no changes to 
>>> Linux, but it involves massive changes to Xen itself and requires 
>>> hardware support: run dom0 as a fully virtualized guest, and assign 
>>> it all the resources dom0 can access.  It's probably a massive effort 
>>> though.
>>>
>>> I've considered it for kvm when faced with the "I want a thin  
>>> hypervisor" question: compile the hypervisor kernel with PCI support 
>>> but nothing else (no CONFIG_BLOCK or CONFIG_NET, no device drivers), 
>>> load userspace from initramfs, and assign host devices to one or more 
>>> privileged guests.  You could probably run the host with a heavily 
>>> stripped configuration, and enjoy the slimness while every interrupt 
>>> invokes the scheduler, a context switch, and maybe an IPI for good 
>>> measure.
>>>     
>>
>> This would be an acceptable model i suspect, if someone wants a 'slim 
>> hypervisor'.
>>
>> We can context switch way faster than we handle IRQs. Plus in a  
>> slimmed-down config we could intentionally slim down aspects of the  
>> scheduler as well, if it ever became a measurable performance issue.  
>> The hypervisor would run a minimal user-space and most of the  
>> context-switching overhead relates to having a full-fledged user-space 
>> with rich requirements. So there's no real conceptual friction between 
>> a 'lean and mean' hypervisor and a full-featured native kernel.
>>   
>
> The context switch would be taken by the Xen scheduler, not the Linux  
> scheduler. [...]

The 'slim hypervisor' model i was suggesting was a slimmed down 
_Linux_ kernel.

	Ingo
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